bug in dreach encoding

Arif reported it. 
https://github.com/dreal/dreal3/blob/master/src/tests/drh/arif.drh

```bash
dReach -k 1 arif.drh
```

It gives us an immediate unsat result because the generated `.smt2` file includes the following:
```lisp
    (= mode_1 2)
    (= mode_1 4)
```
